Royal Bank of Scotland is to pay fines totalling £56m following an IT meltdown in 2012 which locked millions of customers out of their accounts.
The group, which includes NatWest and Ulster Bank, has already paid more than £70m in redress to UK customers as a result of the crash, which it has blamed on "unacceptable weaknesses" in its systems.
The Financial Conduct Authority, which has fined RBS £42m, said the IT failure affected 6.5 million customers.
The Bank of England's Prudential Regulation Authority has issued a separate penalty of £14m in relation to the bank's inadequate systems.
Earlier this month, Ulster Bank was fined €3.5m by the Central Bank for the IT failure here.
Over the course of several weeks in the summer of 2012, many RBS customers could not use online banking facilities or obtain accurate account balances from ATMs.
They were also unable to make mortgage payments on time and customers were left without cash in foreign countries.
Among other problems, the banks applied incorrect credit and debit interest to customers' accounts and produced inaccurate bank statements.
The issues in June 2012 stemmed from a botched upgrade to the software that processed updates to customers' accounts overnight.
When it noticed problems with the upgrade the FCA said the bank's central IT function decided to uninstall it without first testing the consequences of that action.
Tracey McDermott, the FCA's director of enforcement and financial crime, said there were "failures at many levels" to identify and manage the risks stemming from such an incident.
"Modern banking depends on effective, reliable and resilient IT systems. The banks' failures meant millions of customers were unable to carry out the banking transactions which keep businesses and people's everyday lives moving," she added.
RBS chairman Philip Hampton said the bank's IT failure in the summer of 2012 "revealed unacceptable weaknesses in our systems and caused significant stress for many of our customers".
"As I did back then, I again want to apologise to all customers in the UK and Ireland that we let down two and a half years ago," the chairman said.
Mr Hampton said the bank has since spent hundreds of millions of pounds on increasing the resilience of its IT systems.
RBS said 16 staff forfeited pay and bonuses worth around £6m as a result of the incident, including the previously disclosed decision by former RBS chief executive Stephen Hester and Ulster Bank boss Jim Brown to waive any bonus that might have been awarded to them in 2012.
RBS and NatWest have over 26 million UK customers, 4,000 ATMs and 2,120 bank branches. Ulster Bank has 450 ATMs and 90 bank branches in Northern Ireland.
The fine by the UK's PRA was the first since it was formed in April 2013.
The regulator said the incident began on June 18 and directly affected at least 6.5 million customers in the UK, 92% of whom were retail customers.
Disruption to the majority of RBS and NatWest systems lasted until June 26 and in the case of Ulster Bank systems until July 10. Disruptions to other systems continued into July.
"The severe disruption experienced by RBS, NatWest and Ulster Bank in June and July 2012 revealed a very poor legacy of IT resilience and inadequate management of IT risks," Bank of England deputy governor Andrew Bailey said today.
"It is crucial that RBS, Natwest and Ulster Bank fix the underlying problems that have been identified to avoid threatening the safety and soundness of the banks," he added.
